The invention relates to a method for the manufacture of plastic tubes, of the type according to which a tube blank brought to a temperature near its molecular orientation temperature is subjected to biaxial stretching as a result of radial expansion of the tube blank within a forming barrel, the inside diameter of which is equal to the diameter desired for the plastic tube, with the exception of the thermal expansion, with the formation of a blister at one end of the tube and controlled displacement of this blister as far as the other end of the tube, and with longitudinal stretching.
Such a method makes it possible to manufacture molecularly oriented plastic tubes having improved mechanical properties.
GB-A-1,432,539 teaches such a method in a form which, however, proves difficult to employ for pipes of great length. In fact, during pressurization, inflation (formation of the blister) and therefore molecular orientation are often initiated at a plurality of locations at the same time; this phenomenon, which is difficult to control, gives rise to pronounced thickness variations in the longitudinal direction and even to folds where two expansion fronts meet.
U.S. Pat. No. 4,098,857 affords an improvement to the abovementioned bi-orientation method by the use, within a mould, of a sleeve which grips the blank. At the start of the operation, the sleeve restricts radial expansion and is then progressively withdrawn in order to allow the stretching zone to advance.
EP-B-0,072,064 also uses such as a sleeve which grips a tube blank; during manufacture, the sleeve is progressively withdrawn, and a counterpressure system is provided for adjusting the axial displacement of the sleeve which, at one end (in contact with the wall of the tube blank during stretching), is equipped with an annular piston of widened frustoconical shape. The longitudinal stretching of the tube blank is essentially obtained as a result of the friction of the sleeve on the cross-section of the tube during its relative displacement along the tube. Such a solution, although making it possible to control the displacement of the blister, brings frictional forces into action in order to ensure longitudinal stretching; now it is known to be difficult to control accurately the frictional forces between two moving parts, so that longitudinal stretching risks causing appreciable variations along the tube. Moreover, the friction of the sleeve against the outer surface of the plastic tube may cause defects in appearance on this surface.
The object of EP-A-0,404,557 is to provide a positive control of the longitudinal stretching of the tube blank, this control being absent from the method mentioned above. That end of the tube blank which is opposite that where the blister initially forms is locked in a piston which exerts a stretching force on the tube blank, so that friction between tube blank and sleeve is no longer necessary for ensuring such a stretching force. The method according to EP-A-0,404,557 therefore makes it possible to improve uniformity in the degree of longitudinal stretching over the length of the tube.
However, so that the stretching of the plastic tube can take place under good conditions, this tube has to be brought to and maintained at an appropriate temperature, near its molecular orientation temperature, by means of a hot fluid which, according to EP-A-0,404,557, circulates not only in the plastic tube blank, but also around the sleeve. The hot fluid which passes around the sleeve is in direct contact with the inner surface of the forming barrel which is thus maintained at a relatively high temperature.
At the end of radial expansion, according to EP-A-0,404,557, the plastic of the tube blank comes into contact with the hot inner surface of the forming barrel and does not set rapidly, even if, as provided, a cold fluid is circulated within the plastic tube. This relatively slow cooling of the outer layer of the tube contributes to increasing the duration of the forming cycle and to reducing the productivity of the method.
The object of the invention is, above all, to provide a method which overcomes these disadvantages and makes it possible to improve productivity appreciably.
The method according to the invention is characterized in that the radial expansion is carried out in at least two phases, namely:
a first phase, during which the tube blank is inflated to an intermediate diameter determined by a double-walled casing introduced into the forming barrel, with a hot fluid circulating between the two walls of the casing,
and, subsequently, the casing is progressively taken out of the forming barrel in order, during a second phase, to allow the radial expansion of the plastic tube to the inside diameter of the forming barrel, and the longitudinal stretching of the said tube,
and the forming barrel is cooled externally.
The presence of the double-walled casing, along with internal circulation of hot fluid, makes it possible to maintain the temperature of the tube blank, whilst avoiding direct heating of the inner surface of the forming barrel, by virtue of which the duration of the forming cycle is reduced and productivity is increased.
According to another aspect of the invention, which may be used independently or in combination with the preceding aspect, the object of the invention is to make radial expansion uniform along the entire length of the tube, despite the size of this expansion.
Preferably, another object of the invention is to provide a method making it possible to produce a socket at one end of the tube, with a groove for receiving a gasket, whilst ensuring a virtually constant thickness of the tube obtained over its entire length, including in the region of the socket.
The object of the invention, furthermore, is to provide a method which remains relatively simple and economical to put into practice.
This other aspect of the invention is based on an analysis of the phenomenon of expansion of a tube blank as a result of an increase in the internal pressure and on considerations regarding the actual stresses generated in the thickness of the tube. FIG. 1 of the accompanying drawings is a graph illustrating the diameter variations, expressed as a percentage and plotted on the abscissa, of a plastic tube (at a temperature near the molecular orientation temperature) subjected to an internal pressure, the value of which is plotted on the ordinate. It emerges that the diameter variations are broken down into several steps:
a first step, corresponding to a substantially rectilinear ascending part, characterizes a uniform increase in the diameter of the tube to approximately 30% with the increase in the internal pressure;
a second step, in which the stress in the wall passes through a maximum and then descends again very slightly in order to stabilize at a constant value while the diameter increases: this is the blister phenomenon.
This phenomenon may terminate in a third step, in which the value of the actual stress in the wall decreases slightly.
For manufacture, therefore, it is indispensable to raise the internal pressure of the tube to a critical value in order to start a blister, and then stabilize this pressure in order to avoid the rapid bursting of the tube.
In the first step, corresponding to the uniform increase in the diameter of the tube, the circumferential elongation caused by expansion remains below or equal to the yield point of the material of the tube. In the second step, corresponding to the blister phenomenon, the circumferential elongation exceeds the yield point of the material of the tube.
The method according to the second aspect of the invention, for the purpose of making the expansion phenomenon as uniform as possible, is characterized in that the radial expansion is carried out in at least two phases, namely:
a first phase, during which the tube blank is inflated uniformly to an intermediate diameter which is below the inside diameter of the forming barrel and at which the circumferential elongation remains below or equal to the yield point of the material of the tube, this first phase taking place virtually without any longitudinal stretching,
and at least one other phase for changing to the inside diameter of the forming barrel, with longitudinal stretching.
Preferably, the intermediate diameter is determined by introducing into the forming barrel a sliding casing, the inside diameter of which is equal to the intermediate diameter, and, after the first expansion phase, this casing is progressively taken out of the forming barrel in order to allow the second phase of radial expansion.
The longitudinal stretching of the tube, carried out essentially during this second phase, is obtained by locking the two ends of the tube blank in respective clamping means and by moving the means for clamping one end away from the means for clamping the other end of the tube.
Advantageously, a gas under pressure, especially compressed air, is injected between the outer wall of the plastic tube and the inner wall of the casing, in order to make it easier for these two parts to slide relative to one another at the moment when the tube is taken out of the casing and to avoid unwanted friction.
Preferably, a free space remains, within the forming barrel, between that end of the sliding casing which is fully engaged in the barrel and the adjacent end of the barrel, this free space making it possible to produce the blister as a result of an increase in the internal pressure in the tube blank, without the need to displace the sliding casing beforehand.
That end of the plastic tube which is distant from the blister formation zone can be pushed towards this zone at the moment of production of the blister, in order to bring about an increase in the thickness of the material making it possible to obtain a blister having a thickness which is substantially equal to that of the remaining part of the tube.
Advantageously, a socket with a groove for receiving a gasket is produced on the plastic tube during the formation of the blister at one end of the tube.
Preferably, when the wall of the blister comes into contact with the wall of the socket mould, the adjacent end of the tube is pushed towards the socket, in order to obtain a wall thickness in the region of the groove for the gasket which is substantially equal to the thickness of the rest of the socket and of the tube.
Advantageously, the forming barrel is cooled externally by spraying or regulation.
The invention also relates to an installation for carrying out the above-defined method, this installation comprising a forming barrel, in which the tube blank is engaged, and means for shutting off and clamping each end of the tube blank, as well as means for circulating a fluid, especially a liquid, in the tube blank and for varying the pressure of this liquid, and is characterized in that it comprises a double-walled casing and means for circulating a hot fluid, in particular at a temperature near the orientation temperature, in this casing which is mounted slidably in the forming barrel and which, by means of its inside diameter, determines an intermediate expansion diameter, this casing surrounding one end of the tube blank, the said end being provided with clamping means forming a piston mounted slidably in the casing and integral with a cylinder extending beyond the end of the casing, independent means for driving with a sliding action being provided respectively for the casing and for the cylinder equipped with the piston.
The piston mounted at the end of the cylinder preferably comprises passage means for injecting a gas under pressure, especially compressed air, between the outer wall of the plastic tube and the inner wall of the casing and for making it easier for them to slide relative to one another.
Advantageously, the installation comprises, at that end of the forming barrel which is opposite the cylinder equipped with the piston, means for clamping the end of the plastic tube, likewise forming means for shutting off the forming barrel and defining a mould for a socket at the end of the plastic tube.
The mould thus defined for the socket comprises at least two parts which determine a groove for a gasket and which are mounted slidably relative to one another, the part located axially towards the outside of the plastic tube being capable of approaching the other part in order to bring material into the region of the groove for the gasket and making it possible to obtain a wall having a thickness which is substantially constant over the entire length of the socket.
Advantageously, means for detecting the coming of the wall of the socket against the corresponding part of the mould are provided for triggering the displacement of the other part of the mould when contact is made.
The invention also relates to a bi-oriented plastic tube comprising a socket provided with a groove for receiving a gasket, the thickness of this tube being substantially constant over its entire length and in the region of the socket. The properties of the tube are substantially uniform over its entire length.
According to the invention, a bi-oriented plastic tube comprising a socket provided with a groove for receiving a gasket is also characterized in that the relative axial stretching of the socket is greater than the axial stretching of the remaining part of the finished tube.
Preferably, the axial stretching of the socket over its entirety is at least equal to 1.5 times the axial stretching of the remaining part of the tube.